1. Field of the Invention
The present invention generally relates to the determination and control of spacecraft attitude, and more specifically to a method and system for positioning star trackers to produce low error attitude control signals.
2. Description of the Related Art
Spacecraft such as satellites are used for various purposes including scientific research and communications. In many applications the spacecraft must be positioned in particular attitudes, or orientations, in space. Spacecraft attitude includes roll, pitch, and yaw as described in "Spacecraft Attitude Determination and Control", edited by James R. Wertz, D. Reidel Publishing Company, Boston, U.S.A. 1986, pages 1-21. A precision spacecraft attitude determination using gyros and star trackers was described in Wu and Hein, "Stellar Inerial Attitude Determination for LEO Spacecraft," Proc. of the 35 th Conference on Decision and Control, Kobe, Japan, December 1996, pages 3236-3244. In this system, a real-time knowledge of spacecraft attitude is computed by numerically integrating gyro data measuring spacecraft dynamic motion, and star tracker data is processed to generate corrections to attitude and gyro rate bias estimates which are provided by a six-state extended Kalman filter. Star trackers used for this purpose are also described in B. Kunkel et al.,"METOSAT Second Generation Enhanced/Visible/IR Imager SERVER I and its Environmental Monitoring Potential", Proc. of the Central Symposium of the International Space Year Conf., Munich, Mar. 30-Apr. 4, 1992.
As used herein, the term "star tracker" includes all star tracking devices which image or project a star onto a pixel array, including but not limited to gimbaled star trackers, fixed head star trackers and star scanners. Star trackers are currently mounted on spacecraft without considering the direction in which an imaged star traverses their CCD (charge coupled device) pixel arrays. Because of star tracker spatially dependent errors stemming from the spatial position and movement of star images across pixel arrays, the attitude determination performance is sensitive not only to the star tracker temporal noise, but also to the tracked star motion across the detectors. Star tracker pointing error varies positively with this star sensor spatially dependent error.